Sacrificial anode buying guide: choose the right type
Updated: 17.10.2026
You search for a sacrificial anode because one metal part is meant to corrode before the equipment around it does. The same search returns rods for hot-water cylinders, collars for propeller shafts, kits for sterndrives, hanging zincs for docks and plates for pool skimmers. They cannot replace one another. This sacrificial anode buying guide puts the decisions in order: identify the job, match the alloy to the water, confirm every fitting dimension, and only then compare the price. All eleven current listings are on the [sacrificial anode comparison](/hub/hot-water/sacrificial-anode). Measure the old part before you use a shorter table. A qualifying purchase can earn a commission at no extra cost to you.
You search for a sacrificial anode because one metal part is meant to corrode before the equipment around it does.
You search for a sacrificial anode because one metal part is meant to corrode before the equipment around it does. The same search returns rods for hot-water cylinders, collars for propeller shafts, kits for sterndrives, hanging zincs for docks and plates for pool skimmers. They cannot replace one another. This sacrificial anode buying guide puts the decisions in order: identify the job, match the alloy to the water, confirm every fitting dimension, and only then compare the price. All eleven current listings are on the sacrificial anode comparison. Measure the old part before you use a shorter table. A qualifying purchase can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Job | Alloy | Fit | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  AB Tools AB Tools AQ051 | Shaft collar | Zinc | 25 mm shaft | 8.0 | about £27.64 | View offer |
| Preis-Tipp |  DOJA Barcelona DOJA Barcelona M8 250 × 22 mm | Cylinder rod | Magnesium | M8, 250 × 22 mm | 8.2 | about £12.99 | View offer |
 XRmarine XRmarine Volvo Penta 5-pack | Sterndrive kit | Zinc | Volvo part list | 8.7 | about £29.99 | View offer |
On this page
Five anodes, one search word
A cylinder rod sits inside an enamelled hot-water vessel and protects exposed steel. In this set those are three magnesium rods: one described as 16 × 200 mm, and two M8 rods at 250 × 22 mm. A shaft collar clamps around a propeller shaft, so the bore must match: 1-1/8 inch (28.6 mm) on the AQ044 collars, and 25 mm on the AQ051.
A sterndrive kit is identified from the drive and the old part numbers. The XRmarine five-pack lists several Volvo Penta references, which is not proof that every piece fits every drive. A hanging zinc, such as the £125 holama, protects a dock or boat lift by cable. A skimmer zinc is a basket plate, not a cylinder rod or a hull anode.
Write down one plain description, such as “magnesium rod inside my cylinder”, before you open product pages. If you cannot name the job, use the comparison by application rather than the highest star line.
Water before alloy
Once the job is clear, the water decides the material. MG Duff’s UK guidance gives zinc or aluminium for salt water, aluminium for brackish water and magnesium for fresh water. Zinc and aluminium can develop a coating in fresh water. Magnesium can waste rapidly in salt water, and it must not be fitted to a wooden hull.
That marine rule does not mean you replace a cylinder’s magnesium rod with a boat zinc. Vaillant’s UK uniSTOR instructions specify a magnesium sacrificial anode for the enamelled cylinder in that manual, with an external-current alternative for the relevant vessel. The cylinder maker’s documentation takes priority.
If the boat stays in another type of water for more than two weeks, MG Duff advises changing the anodes. For a brackish mooring, aluminium may be the third option, and none of these eleven listings is identified as aluminium. The trade-offs are in the magnesium or zinc anode guide.
Measure the old part
The safest reference is the maker’s part number plus the removed part. For a cylinder rod, record the thread, diameter, length and stud length. M8 is the thread, not the whole fit. UK trade listings show M8 rods at 400 mm in different diameters. The DOJA rods here are 250 × 22 mm, and the shared review block mentions M5 and M6 heaters and studs too short for the plate.
The £8.39 ZEPFJHE title gives 16 × 200 mm and no thread. The colour field says “2 Pack”, and the title does not confirm two rods. A 5.0 from one rating does not supply the thread. For a shaft collar, measure the shaft, not the outside of the old zinc. A 1-1/8 inch shaft is about 28.6 mm, so an AQ044 is not the 25 mm AQ051. For a drive kit, compare each old piece’s bore, thread and shape with the part reference.
Fields that do not describe the anode
Catalogue fields count only when they agree with the title and the maker’s documentation. The single DOJA rod says “Corded Electric” and “Is Electric No”. The pair says 1500 W and “Is Electric Yes”. A plain magnesium rod is not a powered anode, which uses a controller and a non-sacrificial electrode. Do not connect a rod to electricity because an attribute says watts.
The XRmarine page has a bullet that calls the product an ignition coil. Other fields identify five hollow-end anodes. Compare the parts rather than repeating that sentence. The two AQ044 URLs describe the same 28.6 mm collar, share an image and a review, and both show a different ASIN in the technical details, at £5.19 and £26.86. The cheaper URL is not a bargain until the live seller and the supplied part match. The two DOJA ASINs share one 384-review block, so that count is not separate evidence for each pack.
Common questions
Is a boat zinc the same part as a cylinder anode?
No. A boat zinc may clamp to a shaft or bolt to a drive, and the alloy is chosen for that water. A cylinder rod sits inside a particular hot-water vessel and must match the manufacturer’s thread and dimensions. Both are sacrificial metals. They are not interchangeable parts.
Which measurement should I take first?
Start with the maker’s part number, then record the dimensions that control fit. For a rod, that means thread, diameter, length and stud length. For a shaft collar, measure the shaft diameter. For a drive kit, compare every old anode’s shape, bore and fixing.
Does M8 mean a rod fits my cylinder?
No. M8 identifies one thread dimension. Rods with an M8 thread can have different lengths, diameters and stud arrangements, and the access space may also matter. Match the complete specification in your cylinder manual rather than buying from the thread label alone.
Can one anode work in salt and fresh water?
Not automatically. UK marine guidance normally associates zinc or aluminium with salt water, aluminium with brackish water and magnesium with fresh water. A stay of more than two weeks in a different water can require an anode change. Follow the vessel and anode maker’s guidance for your installation.
Why are there two prices for the AQ044?
This extract contains two URLs with the same AQ044 description, at £5.19 and £26.86, and both show a conflicting technical-details ASIN. That is a catalogue warning, not evidence of a bargain. Check the current seller, variation, product reference and supplied contents before ordering either URL.
Does a skimmer zinc protect a hot-water tank?
No. The pool products here are plates fixed in a salt-system skimmer basket. They are not threaded magnesium rods and they are not specified for a cylinder. Use the cylinder manufacturer’s prescribed sacrificial or powered protection rather than adapting a pool accessory.
Related pages
Sources
Public UK sources listed below, and amazon.co.uk listings read on 16 October 2026.
- Vaillant uniSTOR manual (retrieved 16.10.2026)
- Hot Water Association servicing guide (retrieved 16.10.2026)
- GOV.UK competent person schemes (retrieved 16.10.2026)
- Approved Document G (2024 amendments) (retrieved 16.10.2026)
- BS EN 12499:2003 (retrieved 16.10.2026)
- MG Duff anode materials (retrieved 16.10.2026)
- Lochinvar Correx instructions (retrieved 16.10.2026)
- Practical Boat Owner on corrosion protection (retrieved 16.10.2026)